Jet quenching at RHIC and the LHC
Peter Jacobs, LBNL
Radiative energy loss
BDMPS transport coefficient:
Energy loss:
2
qˆ 

DEmed ~  S C R qˆL2
• DE~L2
• DE linearly dependent on color charge CR
• DE ~independent of partonic energy E
At most: logarithmic dependence of DE on E
 need logarithmically large variation of parton (jet)
Winter Workshop, energy to see its evolution
March 12, 2006
Jet Quenching at RHIC and LHC
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Jet quenching at RHIC…
D. d’Enterria
STAR, Phys Rev Lett 91, 072304
Medium-modified fragmentation?
Winter Workshop,
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Jet Quenching at RHIC and LHC
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Response of medium to lost energy?
pTassoc > 2 GeV
4< pTtrig < 6 GeV
pTassoc > 0.15 GeV
cos(Df)
Near-side ridge
correlated with jets?
STAR, Phys Rev Lett 91, 072304
STAR, Phys Rev Lett 95, 152301
High momentum recoil suppressed  low momentum enhanced
Recoil distribution soft and broad ~ thermalized? angular substructure??
Qualitative picture consistent with jet quenching
 quantitative study of dynamics at low pT?
Winter Workshop,
March 12, 2006
Jet Quenching at RHIC and LHC
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Di-hadrons at yet higher pT
STAR preliminary
8 < pT(trig) < 15 GeV/c
• Away-side yield is suppressed but finite and measurable
 set upper bound on energy loss?
• Suppression without angular broadening or modification of
Winter Workshop,
high z fragmentation: why?
March 12, 2006
Jet Quenching at RHIC and LHC
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High pT di-hadrons and geometric bias
Where are the surviving pairs generated?
?
SW quenching weights+geometry+dynamics
Inclusive hadrons: surface bias
Dihadrons: tangential dominates
distance to origin
A. Dainese et al, hep-ph/0511045
angle wrt ray to origin
Winter Workshop,
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Jet Quenching at RHIC and LHC
Dihadrons: ~volume emission?
T. Renk, hep-ph/0602045
trigger direction
6
Jet quenching at RHIC: summary
Jets are quenched in very dense matter: unique probes of the medium
But current picture is largely qualitative:
• leading hadrons: fragmentation and geometric biases
• pT ~2-5 GeV/c: baryon/meson anomaly not fully understood
• no direct evidence yet for radiative energy loss
• where is the radiation? is it also quenched in the medium?
• color charge, quark mass, length dependence?
• role of collisional energy loss?
• response of medium to lost energy?
Future RHIC measurements: new instrumentation and larger datasets
Jet studies at the LHC complement and greatly extend the RHIC
measurements
Winter Workshop,
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Jet Quenching at RHIC and LHC
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Large Hadron Collider at CERN
mid-late 2007: commission 14 TeV p+p
end 2008: first long 5.5 TeV Pb+Pb run
heavy ion running: 4 physics weeks/year
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Jet Quenching at RHIC and LHC
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From RHIC to the LHC…
Heavy ions at LHC:
• hard scattering at low x
dominates particle production
• low x: calculable (CGC)
initial conditions?
• fireball hotter and denser,
lifetime longer than at RHIC
• dynamics dominated by
partonic degrees of freedom
• huge increase in yield of hard
probes
Winter Workshop,
March 12, 2006
LO p+p y=0
(h++h-)/2
p0
√s
=
5500 GeV
200 GeV
17 GeV
LHC
RHIC
SPS
Jet Quenching at RHIC and LHC
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First jet quenching measurement at the LHC:
inclusive hadron suppression
I. Vitev and M. Gyulassy, PRL 89, 252301(2002)
A. Dianese et al., Eur.Phys.J. C38, 461(2005)
Initial gluon density at
LHC ~ 5-10 x RHIC:
qˆ RHIC ~ 10 GeV 2 /fm
qˆ LHC ~ 70 GeV 2 /fm
RHIC vs LHC
But no dramatic effects: RAA (LHC) ~ 0.1-0.2 ~ RAA(RHIC):
inclusive hadrons have limited sensitivity to initial density
 measure jet structure
Winter Workshop,
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Jet Quenching at RHIC and LHC
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The jet landscape for
5.5 TeV Pb+Pb
collisions
Inclusive jet rates very high
g+jet, Z+jet: precision
measurements, but cover only
limited dynamic range
 study of the evolution of
jet quenching must utilize
inclusive jet and multi-jet
measurements
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Jet Quenching at RHIC and LHC
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Jet measurements for LHC heavy ion collisions
High energy jets: fully reconstructable without fragmentation bias(?)
unbiased jet population  comprehensive study of energy loss
(contrast leading particle biases)
Large kinematic reach  evolution of energy loss
New channels: heavy quark jets at high ET, multi-jet events, Z+jet, very
hard di-hadrons,…
Color charge, quark mass dependence over broad range  basic tests
of energy loss mechanisms
Comparison of similar measurements at RHIC + LHC will provide
deep insight
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Jet Quenching at RHIC and LHC
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What is necessary dynamic range?
Rough argument:
dN g
d
 5  10
LHC
dN g
d
 qˆ LHC  7  qˆ RHIC
RHIC
DERHIC ~ few GeV  DELHC ~ 30 GeV
small modification to fragmentation for Ejet>~200 GeV
GLV Calculation (I.Vitev):
Medium-induced gluon
multiplicity saturates at
Ejet> ~100 GeV
I. Vitev, hep-ph/0603010
 need to measure to
ETjet~200 GeV
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Jet Quenching at RHIC and LHC
Ejet (GeV)
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Medium modification of fragmentation
• MLLA: parton splitting+coherence angle-ordered parton cascade
• good description of vacuum fragmentation (PYTHIA)
• introduce medium effects at parton splitting
Borghini and Wiedemann, hep-ph/0506218
pThadron~2 GeV for
Ejet=100 GeV
=ln(EJet/phadron)
Fragmentation strongly modified at pThadron~1-5
GeV even for the highest energy jets
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Jet Quenching at RHIC and LHC
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Sensitivity of fragmentation to medium
properties
A. Morsch, ALICE
EJet=100 GeV:
2.0 0.7 GeV
• largest medium effects for pT~1-5 GeV
• background limits to >~5 (??)
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Jet Quenching at RHIC and LHC
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Jet broadening
Salgado and Wiedemann
kT (tranverse to jet) in jet
cone R=C
jet
kT
Medium-induced broadening at kT~2 GeV/c 
longitudinal momentum ~few GeV/c
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Jet Quenching at RHIC and LHC
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Size: 16 x 26 meters
Weight: 10,000 tons
TOF TRD
HMPID
ITS
PMD
Muon Arm
PHOS
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TPC
Jet Quenching at RHIC and LHC
ALICE
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ALICE Tracking
Silicon Vertex Detector (ITS): 4 cm < r < 44 cm, 6 layers, >6 m2
Time Projection Chamber (TPC): 85 cm < r < 245 cm, L=1.6m, 159 pad rows
Transition Radiation Detector (TRD) 290 cm < 370 cm, 6 layers of 3 cm tracklets
modest solenoidal field (0.5 T)  good pattern recognition
long lever arm  good momentum resolution
small material budget: vertexTPC outer field cage < 0.1 X0
 robust, redundant tracking: 100 MeV to 100 GeV
Momentum resolution
TPC dE/dx
s~5.5-6.5%
~ 5% @ 100 GeV
5 par. fit
107 central Pb
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Jet Quenching at RHIC and LHC
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ALICE Electromagnetic Calorimeter
• upgrade to ALICE
• ~17 US and European institutions
Current expectations:
• 2009 run: partial installation
• 2010 run: fully installed and commissioned
Lead-scintillator sampling calorimeter
Shashlik fiber geometry
Avalanche photodiode readout
Coverage: ||<0.7, Df=110o
~13K towers (DxDf~0.014x0.014)
depth~21 X0
Design resolution: sE/E~1% + 8%/E
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Jet Quenching at RHIC and LHC
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EMCal support rails
average Frenchman
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EMCal: 120 tons, 50 m2
~same area and weight as STAR barrel calorimeter
Jet Quenching at RHIC and LHC
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Kinematic reach of
ALICE+EMCal
104/year for minbias Pb+Pb:
• inclusive jets: ET>200 GeV
• dijets: ET>170 GeV
• p0: pT~75 GeV
• inclusive g: pT~45 GeV
• inclusive e: pT~25 GeV
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What does the EMCal bring to ALICE?
• fast trigger (level 0/1): enhancement of high pT g, p0,
electron and jet statistics by factors 10-60
• significant improvement in jet reconstruction performance
• extension of direct photon measurements at high pT
• electron-tagged heavy quark jets at high ET
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March 12, 2006
Jet Quenching at RHIC and LHC
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ALICE+EMcal in the larger LHC context
We can agree that large statistics and broad kinematic reach are good!
But rate and kinematic reach are not the only issues:
• main fragmentation modifications are at pT<~5 GeV even for the highest
energy jets
• interaction with medium is per definition soft physics
• hadronization effects may be a central issue  particle ID
• how critical are 300 GeV jets?
ALICE+EMCal effectively trade acceptance/rate in favor of robust
tracking and PID over a broad kinematic range
There are significant measurements that ALICE+EMcal cannot do:
3-jet events, forward rapidity (not yet), Z+jet,…
 heavy ion jet measurements must be done by both ALICE and
CMS/ATLAS
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March 12, 2006
Jet Quenching at RHIC and LHC
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Jets reconstruction in heavy ion events
Goal: reconstruct jet independent of details of fragmentation
unbiased measurement of energy loss
50 GeV jet (Pythia) + central Pb+Pb background (Hijing)
• jet structure clearly visible even for modest energy jets
• but large uncertainties in background fluctuations and energy loss
effects  current studies are only a rough sketch
Winter Workshop,
March 12, 2006
Jet Quenching at RHIC and LHC
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Jet reconstruction and heavy ion background
Rcone
Large jet cone integrates large background
 bkgd fluctuations overwhelm jet measurement
Jet energy fraction outside cone R=0.3
Energy in cone R: background and jets
CDF preliminary
Central Pb+Pb
R
D 2  Df 2
R
• Unmodified (p+p) jets: over 80% of energy within R~0.3
• Baseline algorithm to suppress heavy ion background:
Winter Workshop,
small jet conesJetR~0.3,
March 12, 2006
Quenchingtrack
at RHICp
and
LHCGeV/c
T>2
25
# Jets
fraction of evenst with Njets,rec.>1
Jet splitting for small cones
(hard radiation)
R=0.3, pt>2GeV
R=0.3, p >2GeV, N
t
jets,rec.
all particles
charged+em
charged
=2
- input
- highest jet
- second jet
- mid-cone
- sum
Jet Energy [GeV]
Jet Energy [GeV]
Suggests modified kT-type algorithm:
best resolution from summation of small clusters (hot spots)
 study has only just begun…
Winter Workshop,
March 12, 2006
Jet Quenching at RHIC and LHC
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High pT heavy quarks: color charge dependence
Armesto, Dainese, Salgado and
Wiedemann,
PhysRev D71, 054027 (2005)
RD/h
RB/h
Light hadrons dominantly from gluon jets
B-mesons less suppressed even at high pT (quark jets)
 quark vs gluon color charge
Winter Workshop,
March 12, 2006
Jet Quenching at RHIC and LHC
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High pT electrons
Significant electron yield to
pT~25 GeV/c with e/p~0.01
EMCal provides electron
trigger
 reconstruct heavy quark
jet (ETjet~50+ GeV)
Winter Workshop,
March 12, 2006
Jet Quenching at RHIC and LHC
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EMCal: e/h discrimination at high pT
• Geant, all material
• E/p from EMCal/tracking; shower-shape
1/pion efficiency
103
e
h
20 GeV
E/p
electron efficiency
• First look: good hadron rejection at 20 GeV
• Not yet addressed: electron backgrounds
Winter Workshop,
March 12, 2006
Jet Quenching at RHIC and LHC
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Summary
Jet quenching as an experimental observation is well established
But key issues remain open:
• radiative vs collisional?
• quark mass, color charge dependence?
• response of lost energy to medium?
Jet studies in LHC heavy ion collisions provide:
• similar observables for a (presumably) very different physical
system
• huge kinematic and statistical reach, new observables to
elucidate the energy loss mechanisms in detail
• ALICE+EMcal are crucial for full exploitation of jets as a
probe of dense matter
The future is upon us!
Winter Workshop,
March 12, 2006
Jet Quenching at RHIC and LHC
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Extra slides
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March 12, 2006
Jet Quenching at RHIC and LHC
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Direct photons
Not an easy measurement:
• g/p0 < 0.1 for p+p
(better in central Pb+Pb due
to hadron suppression)
g/p0
• QCD bremsstrahlung
photons significant for
pT<50 GeV/c  isolation
cuts
• tricky issue in heavy
ion collisions
Winter Workshop,
March 12, 2006
Pb+Pb
p+p
CERN Yellow Report
Jet Quenching at RHIC and LHC
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